1. Field of the Invention
The present invention relates to an apparatus for controlling the thickness of a synthetic resin film in which upon the formation of a film by a melt extrusion method, ununiformity in thickness of the film is eliminated and controlled so as to make the thickness of the film uniform or the thickness of the film can be locally controlled as required.
2. Related Background Art
Upon the formation of a film by discharging molten resin from a die lip and gradually thinning the thickness of the film, it is well known in the art that in order to eliminate the ununiformity in thickness of the film, temperature distribution of the lip is varied to thermally control the same.
As an example of the above, there has been proposed an apparatus in which many heaters are provided around the lip, the heating power of each of the heaters is varied to locally adjust the heating amount to the resin by which fluidity and extensibility of the resin is changed to adjust the ununiformity in thickness of the film.
The concrete contents thereof are decribed in detail in Japanese Patent Application Nos. 63-143694 and 63-143695 which have been filed by the applicant of the present application.
In a film molding die used in the execution of the above mentioned method, the heating power of each of the electric heaters provided around the lip is varied in accordance with the degree of the ununiformity in thickness of a finished film to eliminate the ununiformity in thickness of the film or to adjust the film thickness. However, there is a tendency that heating-up with the use of the electric heaters is attained easier than cooling-down.
That is, the film thickness should be adjusted and controlled such that in order to make the film thickness thinner, the temperature of a part of the die lip which forms the thick part of the film is increased, while in order to make the film thickness thicker, the temperature of a part of the die lip which forms a thin part of the film is decreased.
In the above mentioned case, in order to rapidly increase the temperature of the die lip, a large amount of power can be applied to the heaters. However, in order to rapidly decrease the temperature of the die lip, what can be done is to turn off the heaters to minimize the heating amount thereof.
Therefore, there is a difference between the increase in temperature of the die lip and the decrease in temperature of the die lip in responsiveness and easiness.
In addition, the heat capacity of the film molding die is great, so that simply through natural cooling, it takes much time to lower the temperature of the die lip to make the film thickness uniform and a time lag in response to heat leads to an industrial loss.
Further, even when only a heater to be controlled is completely turned off, in a case that other heaters around the heater concerned are turned on under the normal control, it takes much time for the heater to be controlled to cool down to a predetermined set temperature influenced by the temperature values of the other heaters therearound.
In a case that the temperature is automatically adjusted with relative temperature to make the film thickness uniform, the increase in temperature is attained easier than the decrease in temperature, so that the set temperature will be gradually increased and hence the temperature of the entire die will be also increased sometimes beyond the appropriate temperature range for molding, which causes a failure to mold the film in the worst case.